The invention relates to the field of aircraft systems that provide information to the pilot concerning aircraft descent rates.
Numerous prior art systems describe methods and devices to warn pilots of an excessive aircraft sink rate condition. Such systems, including those described in U.S. Pat. Nos. 3,715,718; 3,946,358; 3,947,808; 3,958,219; 4,060,793; and 4,551,723, provide a sink rate warning when the sink rate for a given aircraft altitude exceeds a predetermined threshold value. These systems have been generally designed for fixed wing aircraft. Similarly, U.S. Pat. Nos. 4,293,840 and 4,695,013 describe systems that warn of excessive descent rates particularly useful for helicopters.
U.S. Pat. No. 4,302,827 discloses a method of measuring the height of the aircraft above ground and relaying the height information to the pilot by auditory means such that he could be aware that the aircraft was at the best height above ground to flare the aircraft landing.
Although most of these prior art systems provide useful information by alerting the pilot to potentially dangerous excessive sink rate conditions or increasing the pilot's altitude awareness, none of these systems assist the pilot during the final phase of the landing approach (the landing flare) in landing the aircraft at a predetermined desired sink rate.
Under normal landing conditions, most pilots prefer to have their aircraft touch down at a low sink rate of 0-1 ft. per second. Under other conditions, such as a short runway, or conditions that effectively shorten the runway braking distance such as ice, snow, rain or gusty cross winds, the pilot may elect to touch down at a higher sink rate, perhaps 1--3 ft. per second. Touching down at a higher sink rate allows the wheels to more firmly contact the runway pavement, facilitates more effective braking, earlier deployment of spoilers and thrust reversers, and more positive directional control of the aircraft on the runway. Currently, pilots estimate the aircraft sink rate based on visual cues from the landing environment and adjust the sink rate by control inputs to the elevator, usually by means of fore and aft pressure on the control wheel.
On a precision instrument approach, an aircraft arrives 0.5 miles from the runway threshold at an altitude of approximately 200 ft. above ground level. The pilot relies on cockpit instrumentation to arrive at this position under instrument meteorological conditions, and utilizes a combination of external visual cues and cockpit instrumentation during a visual approach. Most precision approaches utilize a 3 glide slope. Therefore, the vertical sink of the aircraft on the glide slope can be determined by the following relationship: EQU Sink rate*=ground speed*.times.sin 3.degree. FNT *Sink rate and ground speed in feet per second.
For an aircraft with an approach speed of 130 knots, the sink rate is 11.4 ft./sec.
On a standard glide slope, the aircraft crosses the end of the runway (runway threshold) at a height of approximately 50 feet. The distance between the landing gear of the aircraft and the runway is referred to hereafter as the runway altitude.
Regardless of the type of approach, the last portion of the landing (50 ft. runway altitude to touch down) generally referred to as the landing flare, is usually performed by the pilot solely with reference on his visual perception. During the flare, the pilot reduces the sink rate of the aircraft to touch down at the desired sink rate.
The pilot relies on his depth perception, visual cues from the landing environment, and previous landing experiences to sense the altitude of the aircraft's wheels above the runway and the sink rate of the aircraft as it nears touch down. The pilot reduces the sink rate of the aircraft to the desired value as the wheels near the runway. If the pilot desires the smoothest landing possible, he will attempt to reduce the sink rate of the aircraft to a very low value (1 ft. per second or less) before the wheels contact the runway. If the pilot desires a firmer touch down, he will arrest the sink rate less, perhaps to value of 2-3 feet per second.
The pilot's eye level at touch down in transport category aircraft is greater than 15 feet above the runway and the aircraft is traveling at a horizontal speed in excess of 100 knots (150 FPS). At this distance and speed, the pilot's ability to consistently judge the exact height of the aircraft's wheels above the runway and the sink rate of the aircraft is limited. This inability to judge relatively small heights (several feet) and small changes in sink rate during touch down are the primary factors leading to aircraft touch downs at undesirable sink rates. This is more likely to occur when a pilot is landing at an unfamiliar airport, at night, or on a sloping runway, since these situations all reduce the familiar visual cues the pilot relies upon to adjust the aircraft's sink rate during touch down. Most touch downs with undesirable sink rates are by no means unsafe or dangerous. However, undesirably firm landings accelerate the wear of landing gear and airframe components and increase aircraft passenger anxiety. Occasionally, hard landings cause structural damage to the airplane. Landings which occur at less than the desired sink rate on a short or slippery runway may prolong the aircraft landing distance and increase the chance of an overrun. Therefore, there is need for a system which will assist the pilot to more accurately adjust the sink rate of the aircraft so he may touch down at the desired sink rate.